Heat transfer materials

ABSTRACT

In a heat pump, e.g. comprising two adsorbers (1, 2), the adsorbers are monoliths having a multiplicity of open cells capable of flow-through of gas or vapor, and having a coating of an adsorbent, e.g. zeolite, for a fluid such as water vapor. Waste heat, e.g. from a vehicle engine, is supplied to heater (3), which heats up a heat transfer fluid pumped around the system by a reversible pump (11). Heat is lost to the ambient air by a cooler (4), and air from ambient, or recirculated air, is cooled by passing over an evaporator (6) for adsorbate fluid such as water, before entering a vehicle passenger compartment.

This application is the U.S. national-phase application of PCTInternational Application No. PCT/GB97/01709.

This invention concerns improvements in heat transfer materials, andmore especially concerns materials and systems suitable for adsorptionheat pumps, particularly but not exclusively for refrigeration.

A single adsorber heat pump system utilises a dried (activated)adsorbent material eg zeolite, in an adsorber vessel, which is separatedby a valve from an evaporator. All air in the system is evacuated, sothat the only gas present is water vapour. If the valve is opened, theadsorption process begins; and the zeolite adsorbs the water.Simultaneously, water in the evaporator vessel evaporates and picks upheat from ambient. In a desorption phase (regeneration), the water inthe zeolite is expelled by taking up heat; by removing heat at thecondenser, the water vapour is cooled down and condensed to liquid waterto be ultimately returned to the evaporator. A single bed system willobviously run in an intermittent fashion, with cooling achieved onlyduring the adsorption phase. For a continuous cooling system, two ormore adsorbers are used, as described in more detail below.

BACKGROUND OF THE INVENTION

The use of solid sorption systems in refrigeration is not new, and wasfirst used commercially in the USA in the 1920's. using silica gel andsulphur dioxide. However, the development of CFCs, and the availabilityof electricity meant that solid sorption refrigeration was abandoned infavour of the vapour compression cycle. Solid sorption systems aregenerating renewed interest once more, however, as CFCs (andalternatives) are causing increasing environmental concern. The phasingout of CFCs as refrigerants was scheduled for the year 2000 by theMontreal Protocol (1987); Germany requires phase-out by 1995, and the ECby 1997. Other heat transfer media for the vapour compression cycle suchas ammonia, hydrocarbons, CO₂ etc have advantages in some circumstancesbut are not viewed as true alternatives. Adsorption heat pumps are beingviewed positively in the fields of heating and air conditioning, and itseems possible that a first commercial use in this current period wouldbe in automotive air conditioning and/or in a residential or commercialspace heating and/or cooling market.

For example U.S. Pat. No. 5,325,916 (UOP), discloses a zeolite-coatedaluminium heat exchanger tube, and U.S. Pat. No. 4,581,049 (Schiedel)and U.S. Pat. No. 5,054,544 (Zeotech) are similar. here remains a needfor improved solid adsorber materials, and for improved systems.

U.S. Pat. No. 4,199,959 (Institute of Gas Technology) teaches an airconditioning apparatus which does not require any heat transfer fluid.Adsorbers for this apparatus are disclosed comprising aluminiumhoneycombs bonded to aluminium sheet. To the extent that the teaching ofthis document can be understood, the physical form of the adsorbersdiffers from that taught in the present invention in that it appearsthat the coating is applied to honeycomb structures whose axis isperpendicular to the gas flow. The invention is stated to be principallyfor low capacity air conditioning, using solar heat for example, thatmay be used in domestic buildings. We do not believe that this systemcould readily be adapted for use on board a vehicle.

The present invention provides an improved heat pump adsorber unit,comprising a heat transfer monolith having a plurality of elongated opencells capable of through-flow of gas or vapour, said cells being coatedinternally with an adsorbent for a refrigerant fluid, and one or morewalls defining a passage for a heat exchange fluid, external to saidmonolith and separated therefrom, the monolith having an effectivethermal conductivity from its geometric centre to said passage of atleast 5 W/mK.

BRIEF SUMMARY OF THE INVENTION

A heat pump system incorporating the novel adsorber preferably operatesusing waste heat, eg from an internal combustion engine, or waste heatfrom a fuel cell system, but can utilise other low grade heat sourcesincluding heat from sunlight. Higher grade heat, eg from combustion ofgas, may, of course, be used. For commercial or industrial applications,waste heat such as from process streams, may be used. It is well knownthat a heat pump system is an energy-efficient method of providingcooling or heating.

Suitable metal monoliths may be made using technology originallydeveloped for manufacturing metal monolithic supports for exhaust gascatalysts, wherein a metal sheet having a metal corrugated sheet bondedthereto, is wound to form a spiral body. Other monoliths may bemanufactured by extrusion of a suitable material to form honeycombblocks with elongated open cells. Such blocks may be fired to increasestability under operating conditions. In the case of metal monoliths,because the adsorbers operate under very much less stringent conditionsthan exhaust gas catalysts, the choice of metals is usefully broadenedto include those of much higher thermal conductivity than the stainlesssteel which is commonly used for catalytic convertors, especially Al orCu. Preferably, the monolith is formed in such a way that its thermalconductivity is maximised. Suitably, therefore, metal-loadedcompositions are used to fill any gaps caused by normal manufacturingprocedures. In general, sintered metal materials do not exhibit adequatethermal conductivity.

The deposition of adsorbent upon the monolith may be carried out by anadaptation of the technology used to coat monoliths for exhaust gascatalysts with alumina or alumina/ceria. If desired, a preliminary orbase coating of, for example, a high surface area alumina or other metaloxide may be applied to the monolith, conveniently using a binder suchas "Dispural", which is a collodal alumina, and the coated monolithbaked or fired to form an adherent base coat. The adsorbent itself, forexample a molecular sieve such as a zeolite, may then be applied as anaqueous slurry typically containing 40 to 50% of zeolite, and using abinder such as "Ludox", which is a colloidal silica, by dipping themonolith into a bath of slurry, or by drenching the monolith using ashower of slurry. If required to build up particular thicknesses ofzeolite, a first coating may be dried and/or baked, and second andpossibly further coatings applied in the same way. Suitable loadings ofadsorbent may be expressed typically as approximately 400-600 g perliter of total monolith volume.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a two-adsorber system;

FIG. 2 is a graphical representation of the effect of cell density andrazing on measured thermal conductivity;

FIG. 3 is a graphical representation of the effect of various supportmaterials on the adsorption heat pump performance;

FIG. 4 is a graphical representation of simulated adsorption heat pumpperformance with measured thermal conductivities;

FIG. 5 is a graphical representation of the effect of adsorbent loadingon the adsorption heat pump performance.

DETAILED DESCRIPTION OF THE INVENTION

The system of the invention is more particularly described withreference to FIG. 1 of the accompanying schematic drawing, which shows atwo-adsorber system, and this will be used to explain the presentinvention.

With reference to the drawing, it is useful to define certain terms; thecooling coefficient of performance, "COP", and the specific coolingpower, "SCP", are defined as follows: ##EQU1## where Δm is the mass ofadsorbate fluid evaporated from the evaporator during a cycle ofoperation, L_(v) is the latent heat of vaporisation, Q_(h) is the heatinput to the heater, M_(ads) is the mass of adsorbent, and t_(c) is thetime of a complete cycle.

Energy consumption by pumps or valve operation is neglected.

Referring to FIG. 1, a two-adsorber thermal wave recuperative adsorptioncycle heat pump is shown. In this case, the adsorbent is NaX zeolite andthe adsorbate is water vapour. The entire system is evacuated of air andthe only gas present is water vapour. Initially, all valves (7-10, 13)are closed.

Initially adsorber 1 is assumed to be cool and saturated with watervapour, while adsorber 2 is hot and free of water vapour. In the firsthalf-cycle the heat transfer fluid (HTF) is pumped by a small reversiblepump (11) in the direction shown by the arrows. Hot fluid (typically250° C.) contacts adsorber 1 and raises its temperature, the fluiditself being partially cooled (this is the recuperative part of thecycle). At an appropriate temperature (selected according to thecondenser pressure) valve 7 is opened and water vapour leaves theadsorber and enters the condenser 5 (cooled by ambient air). In thecondenser. water condenses and loses heat Q_(co) to the externalenvironment (ambient air). The partially cooled HTF (exiting theheating/cooling jacket of adsorber 1) then passes into the cooler (4)where ambient air provides further cooling (to, typically, 40° C.). Thewater which condenses in condenser 5 is collected in a receiver, 12.

The cold HTF then circulates over adsorber 2 (initially hot and free ofwater vapour), and when an appropriate temperature (selected accordingto the evaporator pressure) is reached valve 8 is opened. Liquid wateris allowed to evaporate and the vapour adsorbs onto the zeolite inadsorber 2. The evaporator 6 cools, by virtue of the latent heat ofevaporation, and consequent removal of heat Q_(ev) is used to providethe cooling of the ambient air, or process cold stream. Upon passingover adsorber 2, the initially cool HTF is partially heated(recuperation), it then flows through the heater 3 to attain its maximumtemperature. The hot fluid then flows on to contact adsorber 1, asdescribed above.

After a predetermined time (usually when adsorber 2 is fully saturatedand adsorber I is fully regenerated), the half-cycle time, valves 7 and8 are closed and the heat transfer fluid circulated in the reversedirection. The quantity of water condensed and collected in the receiverduring the first-half cycle is then returned to the evaporator byopening valve 13. The second half-cycle then continues as per the firsthalf-cycle, but with the functions of adsorbers 1 and 2 swapped.

The "pumping" of vapour between the evaporator and condenser (or vapourrecompression) is thus performed by the adsorption/desorption processesoccurring within the adsorbers; i.e., this is what replaces themechanical compressor in a conventional CFC system. The driving forcefor this process is generally energy in the form of waste heat althoughother sources such as gas combustion or solos energy may be used. Incontrast, in a conventional system, the energy must be supplied asmechanical work from an engine, or electricity in the case of anelectrically driven domestic/commercial system.

Summary of heat loads in the system as applied to a motor car: Q_(co)and Q_(c) are provided by ambient air, Q_(h) is provided by waste heatfrom the engine, and Q_(ev) is the cooling load required by the airentering the passenger compartment.

The heat transfer fluid is passed over and/or through each adsorber, buteach adsorber is isolated from the heat transfer fluid, so that there isonly heat transfer between the fluid and the adsorber. Thus, forexample, each adsorber may be provided with a jacket which contains theheat transfer fluid, and generally known procedures and designs toincrease heat transfer between the adsorber and the fluid may be used. Acommercial heat transfer fluid. eg "Dowtherm"™ may be used.

The principles of the adsorber type heat pump are the same no matterwhat type of adsorber is used. However, it is believed that the novelmonolith-supported adsorbers demonstrate improved performance overeither type of adsorber which has previously met practical acceptance,namely the packed bed and the graphite-consolidated bed (as described byGuilleminot et al, Proceedings of the International Adsorption Heat PumpConference, AES-Vol 31, 401-406 (1994)). Thus, performance as measuredby COP and SCP for the packed bed is rather small, for thegraphite-consolidated bed is respectively 0.9 and 35 (W/kg adsorbent)and for the novel monolith adsorbent is respectively approximately 1 andapproximately 250 (W/kg adsorbent) for a stainless steel monolith, whichdoes not have optimum conductivity. It is believed that the improvedperformance is primarily due to the good mass and heat transferproperties of the monolith in comparison with the graphite-consolidatedbed. Although some heat transfer properties of the graphite-consolidatedbed (eg thermal conductivity) may be better than a stainless steelmonolith adsorber, wall heat transfer coefficient of the novel adsorberis far superior.

The novel adsorbers of the invention may conveniently be in the form ofelongated cylinders. Thus, for all cycle times between 500 seconds and1700 seconds, a stainless steel adsorber radius (or thickness if thereis a central passage for heat transfer fluid) of about 2 cm givesoptimum SCP. For such a thickness, the adsorber should be about 0.75 to1.5 meters long, and this may be accommodated by utilising folding, ormore preferably, a number of small adsorbers connected in series.

Further studies and experiment have confirmed the effectiveness of thenovel adsorber of the present invention, and reference is made toaccompanying FIGS. 2 and 5.

Initial studies confirmed that in cylindrical metal monoliths,increasing cell wall thickness improved the measured effective radialthermal conductivity, by increasing the area, or pathway, for heatconduction.

FIG. 2:

This shows the effect of cell density and brazing on the effectiveradial thermal conductivity. Increasing the cell density improves thethermal conductivity by reducing the tortuosity (or length) of the heatconduction pathways within the monolith, this is a significant effect asthe thermal conductivity more than doubles (see top curve) even thoughthe weight of aluminium in the monolith has only increased by ˜15%.

Improving the thermal conductance of the contact points by brazing (orpainting) with aluminium powder has the most dramatic effect, indicatingthat contact resistance between each layer within the monolith has majorinfluence on the overall radial effective thermal conductivity.

The thermal properties of the substrate are very important: monolithsmade of Fecralloy (thermal conductivity ten-times lower than aluminium)gave measured effective thermal conductivities of only 0.26 W/mK(unbrazed) and 0.32 W/mK (brazed). Fecralloy is a standard material formanufacturing metal monolith substrates for catalytic convertors.

FIG. 3:

This shows computer simulation results for the adsorption heat pumpperformance with different substrate materials. As expected the materialwith very low thermal conductivity (Fecralloy) gives poor performance.Although copper has a much higher thermal conductivity than aluminium,its thermal mass (density multiplied by heat capacity) is also higherthese two factors balance each other, resulting in a performance veryclose to that of aluminium.

FIG. 4:

This shows computer simulation results for the effect of measuredeffective thermal conductivity on adsorption heat pump performance. Asexpected, performance improves with increasing thermal conductivity,although (for the given adsorber dimensions) there is no major gain tobe made with thermal conductivities above 5 W/mK.

FIG. 5:

This shows computer simulation results for the effect of adsorbentloading on heat pump performance. Although the COP (thermodynamicefficiency) decreases with increasing loading (due to the increasedthermal mass), the SCP (specific cooling power) does not degradesignificantly. This is important as it means that the total coolingpower of the adsorber can be increased almost linearly with theadsorbent loading.

Measurement Technique:

The effective radial thermal conductivities are measured by a constantheat flux technique. For cylindrical monolith samples a thin electricalcartridge heater is located at the central axis of the sample, with two(or more) thermocouples located at different radial positions betweenthe centre and the outer surface of the monolith. When an electriccurrent is applied to the heater the centre axis temperature rises and atemperature gradient (measured by the thermocouples) is developedbetween the centre and the outer surface of the monolith; thistemperature gradient being a function of the effective radial thermalconductivity of the sample. For a known heat flux (given by the electricpower consumed by the heating element) the temperature gradient ismeasured; using these data the thermal conductivity is calculated.

What is claimed is:
 1. An improved heat pump adsorber unit, comprising aheat transfer monolith having a plurality of elongated open cellscapable of through-flow of gas or vapour, said cells being coatedinternally with an adsorbent for a refrigerant fluid and one or morewalls defining a passage for a heat exchange fluid, external to saidmonolith and separated therefrom, the monolith having an effectivethermal conductivity from its geometric centre to said passage of atleast 5 W/mK.
 2. An adsorber according to claim 1 wherein the monolithis constructed of one of, aluminum copper or carbon.
 3. An adsorberaccording to claim 2, wherein the adsorbent is a zeolite.
 4. An adsorberaccording lo claim 1, wherein the adsorbent is a zeolite or activecarbon.
 5. An adsorber according to claim 1, having 400 to 600g ofadsorbent per liter of monolith volume.
 6. An adsorber according toclaim 1, wherein the monolith has at least 200 cells per square inch (30cells/sq cm).
 7. A heat pump system utilising a heat exchange fluid andincorporating an adsorber according to claim
 1. 8. A heat pump systemaccording to claim 7, wherein the adsorbent coating is a zeolite and therefrigerant fluid is water.
 9. A heat pump system according to claim 7,comprising two adsorbers and utilising a heat transfer fluid to supplyheat to or remove heat from each adsorber.
 10. A heat pump systemaccording to claim 9, constructed and arranged to use waste heat from avehicle engine as a heat input into the system and to provide cooling toair contained in or supplied to a passenger and/or driver compartment.11. A motor vehicle comprising a heat pump system according to claim 10.12. An improved heat pump adsorber unit, comprising a heat transfermonolith having a plurality of elongated open cells capable ofthrough-flow of gas or vapour, said cells 1eing coated internally withan adsorbent for a refrigerant fluid and one or more walls defining apassage for a heat exchange fluid, external to said monolith andseparated therefrom, the monolith having at least 200 cells per squareinch (30 cells/sq. cm) and an effective thermal conductivity from itsgeometric centre to said passage of it least 5 W/mK.